FATTY SUBSTANCES (GENERAL METHODS) 383 soluble fatty acids. Thus the following oils : Cretan Elliotianus, curcas, grape- seed, Macassar, palm-kernel, cacao, coconut, dogfish, dolphin (from the head) have values varying from 87 to 94 ; dolphin oil (from the jaw), spermaceti and wool fat, from 59 to 66. The number for butter is 86-90, and those for the waxes are also comparatively low. 15. Hydroxy-acids The determination of the quantity of hydroxy-acids contained in a fatty substance is effected by Fahrion's method, based on the insolubility of the hydroxy-acids and the solubility of all the fatty acids, in light petro- leum. From 3 to 5 grams of the fatty substance are saponified in the usual way (see 5 : Saponification), the alcohol evaporated, the soap dissolved in 50-70 c.c. of hot water, decomposed in a separating funnel with dilute hydrochloric acid, shaken well with 100 c.c. of petroleum ether (boiling below 80°) and left until the two separate layers are perfectly clear. The aqueous layer is run off and then the petroleum ether, the insoluble hydroxy- acids, which remain adherent to the walls of the funnel, being washed several times with petroleum ether and afterwards dissolved in boiling alcohol. The alcoholic solution is evaporated to dryness in a tared dish and the residue dried at 100° and weighed. This method allows of the determination of the hydroxy-acids produced by the oxidation, either natural or artificial, of an oil or fat. Such a determina- tion has special importance in the analysis of boiled linseed oil and of the so- called blown oils, which are rich in hydroxy-acids. 16. Lactones or Internal Anhydrides The simplest method of determining the content in internal anhydrides of a mixture of fatty acids is based on the following principle : in a mixture of pure insoluble fatty acids it is found that the acid number is equal to the saponification number, so that there is no ester number. If, however, the fatty acids are accompanied by lactonic anhydrides, the saponification number differs from the acid number. This is because the fatty acids are saturated immediately in the cold by potash, whilst the lactones must be boiled with excess of alcoholic potash in order to be neutralised. Hence, to ascertain the content in lactones of a mixture of fatty acids, it is sufficient to determine by the ordinary methods the acid number and the saponification number and, consequently, the ester number. From the latter the content of lactone may be calculated, when the molecular weight from which the ester number is calculated is known (usually the lactone content is calculated as stearolactone). In order that the acid, saponification and ester numbers of the fatty acids may not be confused with the respective numbers for the fatty sub- stances, it has been proposed to call the former: Constant acid number, constant saponification number and constant ester number. EXAMPLE : If a mixture of fatty acids gives the constant acid number 160 and the constant saponification number 195, the constant ester number will be 35,ummy or gelatinous substances, etc.